1. Field of the Invention
The present invention relates to an optical network, and in particular to a network capable of providing fully meshed interconnection of a number of nodes. Such a network might be used, for example, for telecommunications or data communications.
2. Description of Related Art
In a fully interconnected network, each port of a given node is able to obtain direct connection to every other node. Such an interconnection scheme might be implemented by providing direct optical fibre connections between the output ports of a given node and every other node on the network and input port. For example, in a network with 4 nodes, node 1 would have three output ports with direct fibre connections to nodes 2, 3 and 4 respectively, and would have three input ports with further direct fibre connections from the outputs of nodes 2, 3 and 4, with similar patterns of connection for the other nodes. Such an interconnection scheme however suffers from a scalability problem. Adding a new node requires the addition of 2N fibre links, where N is the new total number of nodes. This necessitates a great deal of re-wiring and becomes more and more difficult to achieve as N becomes large.
It has previously been proposed to address the wiring and scalability problems of a full spatial interconnect by using wavelength division multiplexing (WDM) to provide additional xe2x80x9cvirtual pathsxe2x80x9d over a reduced number of physical connections. By using WDM, the number of fibres F required to mesh fully N nodes can be reduced to: F=2N. In practice a little more than the minimum number of fibres may be used to achieve an optimum balance between the wavelength and spatial multiplex. xe2x80x9cA Precompetitive Consortium on Wide-Band All-Optical Networksxe2x80x9d, S B Alexander et al, Journal of Lightwave Technology, Vol 11 No.5/6 May/June 1993, pp 714-732, discloses one example of a meshed network using an Nxc3x97N wavelength router. The router in this example is located at the hub of a star network. While such an arrangement provides the desired degree of interconnection using a simplified topology and with improved scalability, it still suffers a number of disadvantages. In particular such a network lacks resilience, that is there is a serious loss of performance if damage occurs either at the central router, or on any of the lengths of fibre between the nodes and the router.
According to a first aspect of the present invention there is provided an optical network comprising:
a) a plurality of nodes;
b) at least one optical waveguide ring which is connected to the nodes, and which has a duplex structure and comprises:
i) a first set of optical paths which connect transmitter sides of the plurality of nodes; and
ii) a second set of optical paths which connect receiver sides of the plurality of nodes; and
c) a WDM router which has a plurality of inputs and a plurality of outputs and which is connected between the first set of optical paths and the second set of optical paths,
in use, each of N nodes, where N is an integer greater than 1, communicating with any other one of the N nodes on a respective wavelength channel via the said WDM router.
The present invention provides a network which physically has a ring topology, while logically providing a star mesh interconnection. This arrangement is found to provide increased robustness and reliability and also facilitates physical implementation of the network. Such a network might be used, for example, to interconnect the core nodes of the national PSTN system.
Preferably each of the nodes is connected to the ring via a 1xc3x972 switch, the different ring-side ports of the switch being arranged to transmit or receive signals with different respective directions of progagation around the ring. Preferably each node is arranged to control the switch automatically to select the other of the ring-side ports when communication on the ring via one of the ports is interrupted.
In a network embodying the present invention, a transmitter, for example, in a given node can transmit a signal to the router and onto any other node via one of two possible pathsxe2x80x94a first path progagating in a clockwise direction and a second path propagating in the anti-clockwise direction. The robustness of the network can be further improved by connecting each node to the duplex ring via 1xc3x972 switches which can be used to select one or other of the alternative paths. Accordingly, a break in just one path between the router and the node is not sufficient to interrupt communication, since the other path remains available. The switching between alternative paths may be carried out automatically under local control of the node, but might alternatively be implemented in response to remotely generated network management signals. As a further alternative, each node may be connected to transmit/receive signals in both directions. In this case switches for the selection of one of the directions are located centrally at the router and are operated when a break is detected in communications in one direction.
Preferably the ring comprises a first set of N fibres, each fibre being connected to the transmitter output of a respective node, and to a respective one of N input to the router, and a second set of fibres, each fibre of the second set being connected to a respective one of N outputs of the Nxc3x97N router and to a respective receiver input.
Preferably the WDM router comprises a passive wavelength multiplexer/demultiplexer.
The use of a passive router is preferred as providing robustness with minimal control overheads for the network.